switch(n) Tcl Built-In Commands switch(n)
NAME
switch - Evaluate one of several scripts, depending on a given value
SYNOPSIS
switch ?options? string pattern body ?pattern body ...?
switch ?options? string {pattern body ?pattern body ...?}
DESCRIPTION
The switch command matches its string argument against each of the pat-
tern arguments in order. As soon as it finds a pattern that matches
string it evaluates the following body argument by passing it recur-
sively to the Tcl interpreter and returns the result of that evalua-
tion. If the last pattern argument is default then it matches any-
thing. If no pattern argument matches string and no default is given,
then the switch command returns an empty string.
If the initial arguments to switch start with - then they are treated
as options unless there are exactly two arguments to switch (in which
case the first must the string and the second must be the pattern/body
list). The following options are currently supported:
-exact Use exact matching when comparing string to a pattern. This
is the default.
-glob When matching string to the patterns, use glob-style matching
(i.e. the same as implemented by the string match command).
-regexp When matching string to the patterns, use regular expression
matching (as described in the re_syntax reference page).
-nocase Causes comparisons to be handled in a case-insensitive man-
ner.
-matchvar varName
This option (only legal when -regexp is also specified) spec-
ifies the name of a variable into which the list of matches
found by the regular expression engine will be written. The
first element of the list written will be the overall sub-
string of the input string (i.e. the string argument to
switch) matched, the second element of the list will be the
substring matched by the first capturing parenthesis in the
regular expression that matched, and so on. When a default
branch is taken, the variable will have the empty list writ-
ten to it. This option may be specified at the same time as
the -indexvar option.
-indexvar varName
This option (only legal when -regexp is also specified) spec-
ifies the name of a variable into which the list of indices
referring to matching substrings found by the regular expres-
sion engine will be written. The first element of the list
written will be a two-element list specifying the index of
the start and index of the first character after the end of
the overall substring of the input string (i.e. the string
argument to switch) matched, in a similar way to the -indices
option to the regexp can obtain. Similarly, the second ele-
ment of the list refers to the first capturing parenthesis in
the regular expression that matched, and so on. When a
default branch is taken, the variable will have the empty
list written to it. This option may be specified at the same
time as the -matchvar option.
-- Marks the end of options. The argument following this one
will be treated as string even if it starts with a -. This
is not required when the matching patterns and bodies are
grouped together in a single argument.
Two syntaxes are provided for the pattern and body arguments. The
first uses a separate argument for each of the patterns and commands;
this form is convenient if substitutions are desired on some of the
patterns or commands. The second form places all of the patterns and
commands together into a single argument; the argument must have proper
list structure, with the elements of the list being the patterns and
commands. The second form makes it easy to construct multi-line switch
commands, since the braces around the whole list make it unnecessary to
include a backslash at the end of each line. Since the pattern argu-
ments are in braces in the second form, no command or variable substi-
tutions are performed on them; this makes the behavior of the second
form different than the first form in some cases.
If a body is specified as it means that the body for the next pattern
should also be used as the body for this pattern (if the next pattern
also has a body of then the body after that is used, and so on). This
feature makes it possible to share a single body among several pat-
terns.
Beware of how you place comments in switch commands. Comments should
only be placed inside the execution body of one of the patterns, and
not intermingled with the patterns.
EXAMPLES
The switch command can match against variables and not just literals,
as shown here (the result is 2):
set foo "abc" switch abc a - b {expr {1}} $foo {expr {2}} default {expr
{3}}
Using glob matching and the fall-through body is an alternative to
writing regular expressions with alternations, as can be seen here
(this returns 1):
switch -glob aaab {
a*b -
b {expr {1}}
a* {expr {2}}
default {expr {3}} }
Whenever nothing matches, the default clause (which must be last) is
taken. This example has a result of 3:
switch xyz {
a -
b {
# Correct Comment Placement
expr {1}
}
c {
expr {2}
}
default {
expr {3}
} }
When matching against regular expressions, information about what
exactly matched is easily obtained using the -matchvar option:
switch -regexp -matchvar foo -- $bar {
a(b*)c {
puts "Found [string length [lindex $foo 1]] 'b's"
}
d(e*)f(g*)h {
puts "Found [string length [lindex $foo 1]] 'e's and\
[string length [lindex $foo 2]] 'g's"
} }
SEE ALSO
for(n), if(n), regexp(n)
KEYWORDS
switch, match, regular expression
Tcl 8.5 switch(n)
tcl 8.6.0 - Generated Fri Jan 11 15:37:13 CST 2013